MEMS-based fuel cell and methods

ABSTRACT

A fuel cell has a MEMS fuel-cell structure including an anode, a cathode, and an electrolyte, formed on a substrate, a portion of the substrate being removed from beneath the MEMS structure to leave the MEMS structure suspended in membrane form. An opening may extend through the substrate to leave the MEMS fuel-cell structure in a cantilevered configuration, supported by only one edge. Additional openings may be formed to relieve mechanical stress near an edge or edges supporting the MEMS fuel-cell structure, and/or to limit heat-conducting paths. Specially adapted methods are disclosed for fabricating the MEMS-based fuel cell in any of its various configurations.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of co-pending andcommonly assigned application Ser. No. 10/219,507, filed Aug. 14, 2002,the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

[0002] This invention relates to fuel cells and more particularly toMEMS-based fuel-cell structures and related methods.

BACKGROUND

[0003] Various portable devices, such as laptop computers, personaldigital assistants (PDA's), portable digital and video cameras, portablemusic players, portable electronic games, and cellular phones or otherwireless devices, require portable power sources. The weight andinconveniences of single-use batteries and rechargeable batteries havemotivated efforts to replace those power sources for portable use. Thus,there is an increasing demand for light-weight, re-usable, efficient,and reliable power sources in such applications and in many otherapplications as well. In attempts to meet these needs, various portablefuel cells have been developed, such as ceramic-based solid-oxide fuelcells, direct methanol fuel-cell (DMFC) systems,reformed-methanol-to-hydrogen fuel-cell (RMHFC) systems, and otherproton-exchange-membrane (PEM) fuel-cell systems. Microscale designprinciples have been applied to the design of portable fuel cells toprovide improved power density and efficiency and to provide lower cost.There is a continuing need and a large anticipated market for improvedpractical compact portable fuel cells with rapid startup times andimproved efficiency. There is a particular need for compact portablefuel cells with improved thermal isolation of their active portions andwith reduced thermally-induced mechanical stress.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] The features and advantages of the disclosure will readily beappreciated by persons skilled in the art from the following detaileddescription when read in conjunction with the drawings, wherein:

[0005]FIG. 1A is a top plan view and FIG. 1B is a side elevationcross-sectional view illustrating a stage in fabrication of a firstembodiment of a fuel cell made in accordance with the invention.

[0006]FIG. 2A is a top plan view and FIG. 2B is a side elevationcross-sectional view illustrating another stage in fabrication of afirst embodiment of a fuel cell made in accordance with the invention.

[0007]FIG. 3A is a top plan view and FIG. 3B is a side elevationcross-sectional view illustrating another stage in fabrication of afirst embodiment of a fuel cell made in accordance with the invention.

[0008]FIG. 4A is a top plan view and FIG. 4B is a side elevationcross-sectional view illustrating another stage in fabrication of afirst embodiment of a fuel cell made in accordance with the invention.

[0009]FIG. 5 is a flow chart illustrating an embodiment of a method forfabricating a fuel cell in accordance with the invention.

[0010]FIG. 6 is a side elevation cross-sectional view illustrating asecond embodiment of a fuel cell made in accordance with the invention.

[0011]FIG. 7A is a top plan view and FIG. 7B is a side elevationcross-sectional view illustrating a third embodiment of a fuel cell madein accordance with the invention.

[0012]FIG. 8 is a side elevation cross-sectional view illustrating afourth embodiment of a fuel cell made in accordance with the invention.

[0013]FIG. 9 is a side elevation cross-sectional view illustrating afifth embodiment of a fuel cell made in accordance with the invention.

[0014] FIGS. 10A-10C are top plan views illustrating various stages infabrication of a sixth embodiment of a fuel cell made in accordance withthe invention.

[0015] FIGS. 11A-11C are top plan views illustrating various stages infabrication of a seventh embodiment of a fuel cell made in accordancewith the invention.

[0016] FIGS. 12A-12C are top plan views illustrating various stages infabrication of a eighth embodiment of a fuel cell made in accordancewith the invention.

[0017]FIG. 13 is a side elevation view schematically illustratingvarious positional configurations taken by a fuel cell structure inresponse to temperature changes.

[0018]FIG. 14 is a perspective view schematically illustrating variouspositional configurations taken in response to temperature changes byfuel cell structures in an array of fuel cells made in accordance withthe invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0019] Throughout this specification and the appended claims, the term“fuel cell” means a fuel cell in its usual meaning or a battery cellhaving an anode, a cathode, and an electrolyte. The term “MEMS” has itsconventional meaning of a micro-electro-mechanical system.

[0020] For clarity of the description, the drawings are not drawn to auniform scale. In particular, vertical and horizontal scales may differfrom each other and may vary from one drawing to another.

[0021] One aspect of the invention is a MEMS-based fuel cell 10, asshown in the embodiments of FIGS. 4A, 4B, 6, 7A, 7B, 8, and 9, in whichMEMS techniques are used to make an anode 20, a cathode 30, and anelectrolyte 40 in contact with the anode and cathode. The fuel cell maybe made based on a substrate 50 having an opening 60 extending upwardfrom its bottom surface 70 and having a MEMS structure 80 with a portion90 on the top surface 55 of the substrate and another portion 100 thatextends over only part of opening 60. Portion 90 of the MEMS structurehas mechanical and thermal contact with substrate 50.

[0022] A portion 65 of the opening extends through the substrate. Thus,the portion 100 of the MEMS structure that extends over part of theopening forms a cantilever 110, supported along only one edge 120. Insome embodiments, cantilever 110 is supported by the portion 90 of theMEMS structure on the top surface 55 of the substrate 50. A salientportion 130 of substrate 50 may form part of cantilever 110, as shown inFIG. 9.

[0023] While a fuel supply and means for removing excess products of thefuel-cell reaction are needed, conventional fuel supply andexcess-product removal may be used with fuel cells made in accordancewith the invention and therefore will not be described further herein.

[0024] By way of illustration, various embodiments of a MEMS-based fuelcell 10 made in accordance with the invention are described below,beginning with a first embodiment in which the bottom part of cantilever110 is formed by a layer that serves as anode 20, which extends outwardfrom the top surface 55 of substrate 50 and over part of opening 60. Inthis first embodiment, electrolyte 40 forms the middle layer of the MEMSstructure, and cathode 30 forms the top layer, as shown in FIGS. 3A, 3B,4A, and 4B. Thus, anode 20 supports the electrolyte and cathode. In thisand other embodiments, at least one element selected from among anode20, cathode 30, and electrolyte 40 extends over both portions 90 and 100of MEMS structure 80. Thus, in such embodiments, at least one of thethree elements, anode, electrolyte, or cathode (not necessarily thelowest layer), supports the remaining elements. Thus, MEMS structure 80may comprise a cathode layer supported by an electrolyte layer, theelectrolyte layer being supported in turn by an anode layer (FIG. 4B).Or MEMS structure 80 may comprise an anode layer supported by anelectrolyte layer, the electrolyte layer being supported in turn by acathode layer (FIG. 8).

[0025] Alternatively, as described below for another embodiment, asalient portion 130 of substrate 50 may support the anode, electrolyte,and cathode. Several of the embodiments of MEMS structure 80 describedand illustrated herein comprise stacks of layers. The layers stacked,including the three elements, anode, electrolyte, and cathode, may bethin films.

[0026]FIGS. 1A, 1B, 2A, 2B, 3A, 3B, 4A, and 4B illustrate various stagesin fabrication of a first embodiment of a fuel cell made in accordancewith the invention. FIGS. 1A, 2A, 3A, and 4A are top plan views, andFIGS. 1B, 2B, 3B, and 4B are side elevation cross-sectional views.

[0027]FIG. 5 is a flow chart illustrating an embodiment of a method forfabricating a fuel cell in accordance with the invention. In FIG. 5,various steps of the fabrication method are denoted by referencenumerals S10, S20, . . . , S70. For clarity, this description omitsconventional steps known in the art and used for deposition andpatterning of current collectors and of buffer layers (if required)between electrodes and electrolyte, or for application of barrier layersto protect portions of the structure when etching various componentssuch as the anode cathode, electrolyte, and substrate.

[0028] In an overall fabrication method as shown of FIG. 5, a substrate50 is provided (step S10), a portion of which will be removed later, asdescribed below. Substrate 50 is an etchable material suitably stable ata desired temperature of operation. Depending on the operationtemperature range, the substrate material may be a semiconductor (e.g.,silicon), a metal (e.g., stainless steel), an oxide (e.g., titaniumdioxide), a ceramic (e.g., alumina), a plastic or solid polymer (e.g.,polytetrafluoroethylene). Substrate 50 may be a silicon wafer, forexample, as used in conventional semiconductor integrated circuitfabrication. Three fuel-cell components (anode 20, cathode 30, andelectrolyte 40) are deposited in steps S20, S40, and S30 respectively.If necessary, the anode 20 is patterned (step S25). Similarly, thecathode 30 and/or electrolyte 40 may be patterned (steps S45 and S35respectively).

[0029] The order of depositing these three components (i.e., thesequence of performing steps S20, S30, and S40) may be varied forfabricating various embodiments of the fuel-cell structure. Forfabricating the first embodiment illustrated in FIG. 4B, anode 20 isdeposited on substrate 50 first (step S20) and patterned (step S25,FIGS. 1A and 1B), then electrolyte 40 is deposited on anode 20 (stepS30) and patterned (step S35) as shown in FIGS. 2A and 2B, and thencathode 30 is deposited on electrolyte 40 (step S40) and patterned (stepS45, FIGS. 4A and 4B). If one or more of the fuel-cell elements, e.g.,electrolyte 40, is deposited through a mask, a separate patterning step,e.g., step S35, is not needed.

[0030] A portion of substrate 50 under the three fuel-cell components isremoved (step S50) to leave a portion 100 of MEMS structure 80 supportedin membrane form. An opening 60 is also formed in substrate 50 (stepS60) in at least partial alignment with MEMS structure 80. Steps S50 andS60 can be performed simultaneously or combined into one step (S65).Those skilled in the art will recognize that any known conventionalmethod may be used to align features on one side of substrate 50 withfeatures on the other side, e.g., reflective alignment optics orinfrared alignment through an infrared-transparent substrate.

[0031] As opening 60 is formed, a portion 65 of opening 60 may beextended though substrate 50, to leave a portion 100 of MEMS structure80 supported in cantilever form. In steps S50 and S60 or the combinedstep S65, a salient portion 130 of substrate 50 may be left to provideadditional mechanical support. See FIG. 9, described below.

[0032] Electrical connections (not shown) are made (step S70) at leastto anode 20 and cathode 30. Step S70 may be performed by depositingseparate terminal electrodes in electrical contact with anode 20 andwith cathode 30. These may be conventional conductive terminal pads asused in semiconductor integrated circuits, for example. If fuel-cellstructures are arranged in a stack of layers, the electrical connectionsmay include vias connecting with fuel-cell structures on various layersof the stack.

[0033] As shown in FIGS. 6, 7A, and 7B, embodiments may be made in whichthe fuel-cell element extending over both of the portions 90 and 100 ofMEMS structure 80 is the electrolyte 40. For such embodiments,electrolyte 40 is any suitable solid electrolyte. Examples of suitablesolid-electrolyte materials are cubic fluorites such as Sm- or Gd-dopedCeO₂ and yttria-stabilized zirconia (YSZ, e.g., 8 mole % yttria), dopedperovskite oxides such as La_(0.9)Sr_(0.1)Ga_(0.8)Mg_(0.2)O₃,proton-conducting perovskites such as BaZrO₃, SrCeO₃, and BaCeO₃, otherproton-exchange ceramics, or ion-conductive polymers such as aperfluorosulfonic acid resin membrane (e.g., Nafion™, available fromDuPont Chemicals, Inc., Wilmington, Del.). FIG. 6 shows a secondembodiment and FIGS. 7A and 7B show a third embodiment.

[0034] In the third embodiment, illustrated in FIGS. 7A and 7B,electrolyte 40, while supporting anode 20 and cathode 30, is the middlelayer of MEMS structure 80. Thus, both cathode 30 and anode 20 aresupported by a solid electrolyte layer. Also, in this embodiment, anode20 and cathode 30 are suitably patterned to provide interleavedelectrodes, spaced apart from each other. Both anode and cathode arepatterned thin films in contact with the same side of the solidelectrolyte layer, but spaced apart from each other. Such interleavingof electrodes may also be employed in other embodiments.

[0035] Thus, the fuel cell structure may be made with a cathode and/oran anode that is a patterned thin film in contact with a solidelectrolyte layer. Each of the anode and cathode may be a thin film incontact with opposite sides of the electrolyte or in contact with thesame side of the electrolyte.

[0036]FIG. 8, a side elevation cross-sectional view, shows a fourthembodiment, in which the fuel-cell element extending over both of theportions 90 and 100 of MEMS structure 80 is the cathode 30.

[0037]FIG. 9 is a side elevation cross-sectional view illustrating afifth embodiment of fuel cell 10. As shown in FIG. 9, a salient portion130 of substrate 50 supports the anode 20, cathode 30, and electrolyte40 of fuel cell 10. Salient portion 130 is patterned and etched toprovide openings 131 extending through portion 130 to allow passage offuel and/or an oxidant such as air. Openings 131 may be formedsimultaneously with opening 65. If substrate 130 is sufficiently porouswithout them, then of course openings 131 are not needed.

[0038] As pointed out above, the sequence of steps may be varieddepending on the application and on the details of the desired fuel-cellstructure. Thus, the fuel-cell structure may be formed by depositing anelectrolyte upon the substrate, depositing and patterning an anode uponat least the electrolyte, (optionally) depositing an electrolyte overthe anode, and depositing and patterning a cathode on at least theelectrolyte, the cathode being spaced from the anode. Alternatively, thefuel-cell structure may be formed by depositing an electrolyte upon thesubstrate, depositing and patterning a cathode upon at least theelectrolyte, (optionally) depositing an electrolyte over the cathode,and depositing and patterning an anode upon at least the electrolyte,the anode again being spaced from the cathode. In another alternativesequence, the fuel-cell structure may be formed by depositing andpatterning an anode upon the substrate, depositing an electrolyte uponat least the anode, depositing and patterning a cathode upon at leastthe electrolyte, with the cathode again being spaced from the anode. Inyet another alternative sequence, the fuel-cell structure may be formedby depositing and patterning a cathode upon the substrate, depositing anelectrolyte upon at least the cathode, depositing and patterning ananode upon at least the electrolyte, again with the anode being spacedfrom the cathode. When the anode and cathode are to be interleaved onboth sides of the electrolyte, as shown in FIGS. 7A and 7B, multipledepositions and/or patterning steps may be needed to complete all theanode and cathode patterns.

[0039] A mechanical stress-relief feature may be provided by forming oneor more openings contiguous with both of the portions 90 and 100 of MEMSstructure 80. The openings of the stress-relief feature may extenddownward from the substrate top surface 55 and may extend throughsubstrate 50 to its bottom surface. Such openings also provide a degreeof thermal isolation by limiting heat-conducting paths. By way ofillustration, some examples of such stress-relief features are shown inFIGS. 10A-10C, 11A-11C, and 12A-12C, each of which has a number ofelongated openings.

[0040] FIGS. 10A-10C are top plan views illustrating various stages infabrication of a sixth embodiment of a fuel cell made in accordance withthe invention. Openings 150 are formed along edges of opening 60 so thatthey are contiguous with portions 90 and 100 of MEMS structure 80. Asshown, openings 150 are elongated in a direction parallel to the topsurface of substrate 50 and generally parallel to edges of opening 60.

[0041] FIGS. 11A-11C are top plan views of various stages in fabricationof a seventh embodiment. Openings 160 are elongated in a directionparallel to the top surface of substrate 50 and generally perpendicularto edges of opening 60, while openings 170 are elongated in a directiongenerally parallel to edges of opening 60.

[0042] FIGS. 12A-12C are top plan views illustrating various stages infabrication of an eighth embodiment. In this embodiment, openings 160are similar to those of FIGS. 11A-11C. In the embodiment shown in FIGS.12A-12C, openings 160 for mechanical stress relief are formed adjacentto the one edge by which MEMS structure 80 is supported. Another opening180 extends around three sides of MEMS structure 80, generally parallelto three edges of opening 60 and effectively merged with that portion 65of opening 60 that extends completely through substrate 50.

[0043]FIG. 13 is a side elevation view schematically illustratingvarious positional configurations 220 and 230 that may be taken by afuel cell structure in response to temperature changes during operationof the fuel cell, due, for example, to differences in thermal expansioncoefficient among the cathode, anode, electrolyte, and interconnects.Thus, a portion of MEMS structure 80 has an edge 210, free to move inresponse to heat to reduce mechanical stress that would otherwise occurif MEMS structure 80 were completely fixed.

[0044]FIG. 14 is a perspective view schematically illustrating variouspositional configurations 220, 230, and 240 taken in response totemperature changes by MEMS fuel-cell structures 80 in an array 250 offuel cells made in accordance with the invention.

[0045] Thus, one aspect of the invention is provision of an electricalenergy source comprising a combination of a MEMS structure includingmeans for producing electrical current in an electrolyte with means forsupporting the MEMS structure and means for cantilevering the MEMSstructure from its means of support. Means for conducting heat away fromthe energy source may be provided. A portion of the MEMS structurehaving thermal contact with a substrate may be provided for conductingaway heat generated in the MEMS structure. A portion of the MEMSstructure may have one or more openings provided for relief ofmechanical stress and/or for limiting heat-conducting paths.

[0046] Another aspect of the invention is a fabrication method speciallyadapted for fabricating such an energy source. This specially adaptedmethod encompasses embodiments employing steps of providing a substrate,forming a MEMS fuel-cell structure on the substrate by performing thesubsteps of depositing an electrolyte, depositing and patterning ananode, optionally depositing additional electrolyte, depositing andpatterning a cathode, and removing a portion of the substrate under thefuel-cell structure. An opening through the substrate may be formedadjacent to the MEMS fuel-cell structure while leaving the MEMSfuel-cell structure supported by only one edge, whereby the MEMSfuel-cell structure is cantilevered. Additional openings may be formedfor stress relief and/or for limiting heat-conducting paths.

INDUSTRIAL APPLICABILITY

[0047] The present invention is useful in the manufacture of fuel cells.Fuel cells made by methods of the invention and electronic devicesincorporating such fuel cells are applicable in many electronicapplications, especially those requiring portable devices.

[0048] Although the foregoing has been a description and illustration ofspecific embodiments of the invention, various modifications and changesthereto can be made by persons skilled in the art without departing fromthe scope and spirit of the invention as defined by the followingclaims. For one example, a current collector may be included in a fuelcell made in accordance with the invention, such a current collector maybe porous, and such a current collector may be used (by being depositedand optionally patterned first) to support the other elements of thefuel-cell structure, analogously to the embodiments in which an anode,electrolyte, or cathode supports the other elements. For anotherexample, various fuel-cell structures in an array of fuel cells may havediffering configurations adapted to facilitate their electricalinterconnection and/or their fuel supplies and/or their response to heatgenerated. It is intended that the described and illustrated embodimentsbe considered exemplary only and that the true scope and spirit of theinvention be defined by the following claims.

What is claimed is:
 1. A fuel cell comprising in combination: asubstrate having a substrate top surface, a substrate bottom surface,and an opening, said opening extending upward from said substrate bottomsurface; and a MEMS structure at least partially aligned with saidopening, said MEMS structure including a first portion disposed on saidsubstrate top surface and a second portion extending over only part ofsaid opening, said MEMS structure comprising an anode, a cathode, and anelectrolyte in contact with said anode and cathode.
 2. The fuel cell ofclaim 1, wherein at least a portion of said opening extends through saidsubstrate.
 3. The fuel cell of claim 1, wherein said second portion ofsaid MEMS structure forms a cantilever supported by said first portionof said MEMS structure.
 4. The fuel cell of claim 2, wherein saidcantilever comprises a salient portion of said substrate.
 5. The fuelcell of claim 4, wherein said salient portion of said substrate supportsthe anode, cathode, and electrolyte of the fuel cell.
 6. The fuel cellof claim 4, wherein said salient portion of said substrate includesopenings extending through said salient portion.
 7. The fuel cell ofclaim 4, wherein said salient portion of said substrate is porous. 8.The fuel cell of claim 1, wherein at least one element selected fromsaid anode, cathode, and electrolyte extends over both said first andsecond portions of said MEMS structure.
 9. The fuel cell of claim 8,wherein said at least one element extending over both of said first andsecond portions of said MEMS structure supports the remaining elementsof said electrolyte, anode, and cathode.
 10. The fuel cell of claim 8,wherein said at least one element extending over both of said first andsecond portions of said MEMS structure is the anode.
 11. The fuel cellof claim 8, wherein said at least one element extending over both ofsaid first and second portions of said MEMS structure is the cathode.12. The fuel cell of claim 8, wherein said at least one elementextending over both of said first and second portions of said MEMSstructure is the electrolyte.
 13. The fuel cell of claim 8, wherein saidat least one element extending over both of said first and secondportions of said MEMS structure comprises at least two elements selectedfrom said, anode, cathode, and electrolyte.
 14. The fuel cell of claim1, wherein said MEMS structure comprises a stack of layers, said layersincluding said electrolyte, anode, and cathode.
 15. The fuel cell ofclaim 14, wherein said stack of layers comprises a stack of thin films.16. The fuel cell of claim 1, wherein said MEMS structure comprises acathode layer supported by an electrolyte layer, said electrolyte layerbeing supported in turn by an anode layer.
 17. The fuel cell of claim 1,wherein said MEMS structure comprises an anode layer supported by anelectrolyte layer, said electrolyte layer being supported in turn by acathode layer.
 18. The fuel cell of claim 1, wherein both said cathodeand anode are supported by a solid electrolyte layer.
 19. The fuel cellof claim 18, wherein said cathode comprises a patterned thin film incontact with said solid electrolyte layer.
 20. The fuel cell of claim18, wherein said anode comprises a patterned thin film in contact withsaid solid electrolyte layer.
 21. The fuel cell of claim 18, whereinboth said anode and cathode comprise patterned thin films in contactwith said solid electrolyte layer.
 22. The fuel cell of claim 18,wherein each of said anode and said cathode comprises a thin film incontact with opposite sides of said solid electrolyte layer.
 23. Thefuel cell of claim 18, wherein each of said anode and said cathodecomprises a patterned thin film in contact with the same side of saidsolid electrolyte layer, said anode and cathode being spaced apart fromeach other.
 24. The fuel cell of claim 23, wherein said anode and saidcathode are suitably patterned to interleave with each other.
 25. Thefuel cell of claim 1, further comprising a mechanical stress-relieffeature, said stress-relief feature comprising at least a second openingcontiguous with both of said first and second portions of said MEMSstructure.
 26. The fuel cell of claim 25, wherein said at least secondopening of said stress-relief feature extends downward from saidsubstrate top surface.
 27. The fuel cell of claim 25, wherein said atleast second opening of said stress-relief feature extends through saidsubstrate.
 28. The fuel cell of claim 25, wherein said at least secondopening of stress-relief feature comprises an opening elongated in adirection parallel to said substrate top surface.
 29. The fuel cell ofclaim 25, wherein said stress-relief feature comprises a series ofopenings extending through said substrate, each opening of said seriesof openings being disposed contiguous with said second portion of saidMEMS structure.
 30. An electronic device comprising the fuel cell ofclaim
 1. 31. A fuel cell comprising in combination: a substrate having asubstrate top surface, a substrate bottom surface, and an opening, saidopening extending upward from said substrate bottom surface; and a MEMSstructure at least partially aligned with said opening, said MEMSstructure including a first portion disposed on said substrate topsurface and a second portion extending cantilevered over only part ofsaid opening, said second portion of said MEMS structure comprising apatterned thin film anode, a patterned thin film cathode, and a solidelectrolyte in contact with said patterned thin film anode cathode. 32.The fuel cell of claim 31, wherein said substrate comprises a materialsuitably stable at a desired temperature of operation, said materialbeing selected from the list consisting of semiconductors, metals,oxides, ceramics, plastics, and solid polymers.
 33. The fuel cell ofclaim 31, wherein said substrate comprises silicon.
 34. An electricalenergy source comprising in combination: a MEMS structure, said MEMSstructure comprising means for producing electrical current in anelectrolyte for delivering said electrical energy; means for supportingsaid MEMS structure; and means for cantilevering said MEMS structurefrom said means for supporting said MEMS structure.
 35. The electricalenergy source of claim 34, wherein said electrical-current-producingmeans includes an anode, a cathode, and said electrolyte disposed incontact with said anode and cathode.
 36. The electrical energy source ofclaim 34, further comprising means for conducting heat away from saidelectrical-current-producing means.
 37. A fuel cell comprising incombination: a substrate having a substrate top surface, a substratebottom surface, and an opening, said opening extending upward from saidsubstrate bottom surface; and a MEMS structure at least partiallyaligned with said opening, said MEMS structure including a first portiondisposed adjacent to said substrate top surface and a second portionextending over only part of said opening, said MEMS structure comprisingan anode, a cathode, and an electrolyte in contact with said anode andcathode, said first portion of said MEMS structure having thermalcontact with said substrate for conducting away heat generated in saidsecond portion of said MEMS structure and said first portion of saidMEMS structure having one or more openings for relief of mechanicalstress, said second portion of said MEMS structure having an edge freeto move in response to said generated heat to reduce said mechanicalstress.
 38. An electronic device comprising the fuel cell of claim 37.39. A method for fabricating a MEMS fuel cell, said method comprisingthe steps of: a) providing a substrate; b) forming a fuel-cell structureon said substrate by depositing in suitable order and optionallypatterning an anode, an electrolyte, and a cathode; c) removing aportion of said substrate under said fuel-cell structure to leave saidfuel-cell structure supported in membrane form; and d) forming at leasta first opening adjacent to said fuel-cell structure while leaving saidfuel-cell structure supported by only one edge, whereby said fuel-cellstructure is cantilevered.
 40. A fuel cell made by the method of claim39.
 41. An electronic device comprising the fuel cell of claim
 40. 42.The method of claim 39, wherein said fuel-cell-structure-forming step(b) is performed by performing the substeps of: i) depositing andoptionally patterning an electrolyte upon said substrate; ii) depositingand patterning an anode upon at least said electrolyte; iii) optionallydepositing an electrolyte over said anode; and iv) depositing andpatterning a cathode upon at least said electrolyte, said cathode beingspaced from said anode.
 43. The method of claim 39, wherein said fuelcell-structure-forming step (b) is performed by performing the substepsof: i) depositing and patterning an anode upon said substrate; ii)depositing and patterning a cathode upon said substrate; iii) depositingand patterning an electrolyte over said cathode and anode; and iv)depositing and patterning an anode upon at least said electrolyte v)depositing and patterning a cathode upon at least said electrolyte, saidcathode being spaced from said anode.
 44. The method of claim 39,wherein said fuel-cell-structure-forming step (b) is performed byperforming the substeps of: i) depositing and optionally patterning anelectrolyte upon said substrate; ii) depositing and patterning a cathodeupon at least said electrolyte; iii) optionally depositing anelectrolyte over said cathode; and iv) depositing and patterning ananode upon at least said electrolyte, said anode being spaced from saidcathode.
 45. The method of claim 39, wherein saidfuel-cell-structure-forming step (b) is performed by performing thesubsteps of: i) depositing and patterning an anode upon said substrate;ii) depositing and optionally patterning an electrolyte upon at leastsaid anode; iii) depositing and patterning a cathode upon at least saidelectrolyte, said cathode being spaced from said anode.
 46. The methodof claim 39, wherein said fuel-cell-structure-forming step (b) isperformed by performing the substeps of: i) depositing and patterning acathode upon said substrate; ii) depositing and optionally patterning anelectrolyte upon at least said cathode; iii) depositing and patterningan anode upon at least said electrolyte, said anode being spaced fromsaid cathode.
 47. The method of claim 39, further comprising the stepof: e) forming at least one second opening adjacent to said one edge toprovide mechanical stress relief.
 48. A fuel cell made by the method ofclaim
 47. 49. An electronic device comprising the fuel cell of claim 48.50. The method of claim 47, wherein said at least one second opening isformed to extend through said substrate.
 51. The method of claim 47,wherein said second-opening-step is performed by forming a plurality ofelongated openings.
 52. The method of claim 51, wherein said pluralityof elongated openings are formed to extend through said substrate.
 53. Amethod for fabricating a MEMS fuel cell, said method comprising thesteps of: a) providing a substrate; b) forming a fuel-cell structure onsaid substrate by performing the substeps of: i) depositing andoptionally patterning an electrolyte upon said substrate; ii) depositingand patterning an anode upon at least said electrolyte; iii) optionallydepositing and optionally patterning an electrolyte over said anode; andv) depositing and patterning a cathode upon at least said electrolyte,said cathode being spaced from said anode; c) removing a portion of saidsubstrate under said fuel-cell structure; and d) forming at least afirst opening adjacent to said fuel-cell structure while leaving saidfuel-cell structure supported by only one edge, whereby said fuel-cellstructure is cantilevered.
 54. The method of claim 53, furthercomprising the step of: providing electrical connections.
 55. The methodof claim 54, wherein the step of providing electrical connections isperformed by depositing terminal electrodes in electrical contact withsaid anode and cathode.
 56. A fuel cell made by the method of claim 55.57. An electronic device comprising the fuel cell of claim
 56. 58. Afuel cell assembly comprising a plurality of the fuel-cell structures ofclaim 56, arranged in an array.